Low-loss ultrafast and nonvolatile all-optical switch enabled by all-dielectric phase change materials

Summary All-optical switches show great potential to overcome the speed and power consumption limitations of electrical switching. Owing to its nonvolatile and superb cycle abilities, phase-change materials enabled all-optical switch (PC-AOS) is attracting much attention. However, realizing low-loss and ultrafast switching remains a challenge, because previous PC-AOS are mostly based on plasmonic metamaterials. The high thermal conductance of metallic materials disturbs the thermal accumulation for phase transition, and eventually decreases the switching speed to tens of nanoseconds. Here, we demonstrate an ultrafast switching (4.5 ps) and low-loss (2.8 dB) all-optical switch based on all-dielectric structure consisting of Ge2Sb2Te5 and photonic crystals. Its switching speed is approximately ten thousand times faster than the plasmonic one. A 5.4 dB on-off ratio at 1550 nm has been experimentally achieved. We believe that the proposed all-dielectric optical switch will accelerate the progress of ultrafast and energy-efficient photonic devices and systems.

Phase-change all-optical switches based on chalcogenide materials attract massive attention for its nonvolatile property and cycling ability. It is always a structure of chalcogenide compounds coupling with plasmonic metamaterials (Gholipour et al., 2013(Gholipour et al., )(chen et al., 2013, or chalcogenide compounds metasurface (Choi et al., 2019) (Zhu et al., 2021). Basically, chalcogenide compound materials are widely exploited in non-volatile optical and electronic data storage, and in recent years in displays as well (Wełnic and Wuttig, 2008) (Wuttig et al., 2017) . As the most typical Chalcogenide compound materials, Ge 2 Sb 2 Te 5 (GST) can be reversibly switched between two stable states, the amorphous state and the crystalline one. Different from most solids in which the amorphous and crystalline states have very similar optical property, phase change materials present a pronounced contrast of refractive index and extinction coefficient (Kohara et al., 2006) (Feinleib et al., 1972) (Akola and Jones, 2007) (He et al., 2020). The reversible phase transition can be induced by either programmed laser pulses or electric pulses. It is reported that GST can be switched for 10 12 times between two states, and can store the information for nearly 10 years in room temperature (Nardone et al., 2012) (Lu et al., 2013) (Kim et al., 2016).
Because of surface plasmon resonance, the artificial metal arrays, which show surface frequency-selectivity property and perfect absorbance, have been investigated for light tuning and color printing (Shu et al., 1 ll OPEN ACCESS 2014) (Ahmadivand et al., 2017) (Tittl et al., 2015) (Tian and Li, 2016) (Kumar et al., 2012). However, because it is based on metallic metamaterials, high loss of metal increases the device loss and high conductance of metal weakens the thermal accumulating for the transition process, which may have negative effect on the device switching speed (Kuznetsov et al., 2016). Recently, the tunable all-dielectric metasurface has been explored (Staude et al., 2018) (Sautter et al., 2015) and used in all-optical switches (Zhang et al., 2019). However, most of the previous studies are based on nano-grating structure, which is sensitive to polarization (Karvounis et al., 2016) (Gholipour et al., 2018).
In this work, we proposed a novel all-optical switch, which combines all the aforementioned approaches. It is claimed that all-dielectric metamaterials have lower losses, which means that the power consumption can be remarkably decreased if we make them into switches (Boltasseva and Atwater, 2011). Dielectric materials can also decrease the thermal loss because of its lower thermal conductance. Hence, an all-optical switch based on chalcogenide phase-change materials and photonic crystals (PhCs) with ultrahigh switching speed and low loss has been investigated. It is beneficial for improving switching performance for PC-AOS. We first present our theoretical design for our device concept. Then, we give a typical example of the fabricated optical switch at 1550 nm wavelength. Finally, we describe our demonstration of all-optical reversible switching in the femtosecond regime.

Proposed phase change all-optical switch
Our design of the PhCs on silicon substrate is sketched in Figure 1A. It consists of a thin-film of PhCs, a 25 nm SiO 2 film, a 15 nm GST film and a 100 nm ZnS/SiO 2 film. Phase change materials (GST-225 in this work) act as the dynamic component, which is sandwiched on silicon PhCs. There are dramatically large differences in optical constant between the long-range disorder amorphous phase and long-range order crystalline phase. To control the reversible phase-change processes we formed trains of optical pulses of different duration, typically consisting of a few tens' femtosecond laser pulses. It manifests as changes in transmittance intensity and shifts in the spectrum ( Figure 1B). PhCs have already been realized to limit the electromagnetic wave inside the optical bandgap (Fano, 1961). As shown in Figure 1C, the electric field at the resonance frequency is localized in a single Si layer with PhCs structure. This phenomenon called Fano resonance is because of the interactions of narrow Bragg resonances with broad Mie or Fabry-Pé rot bands in photonic crystals (Luk'yanchuk et al., 2010). Fano resonance can provide a quite steep and asymmetric transmission spectrum (Hayashi et al., 2016) (Yu et al., 2016), which is beneficial for some sensing devices. Similar to hybrid plasmonic metamaterials (Wang et al., 2015) (Nikolaenko et al., 2010), the size parameters and the refractive index (or dielectric constant) of PhCs surface can influence the resonance frequency (Petronijevic and Sibilia, 2016). Thus, by tuning the structure dimension and surface refractive index, the spectrum of the device may red-or blue-shift so that the selection of the switching wavelength and the switching function can be finely realized.
Figures 2A and 2B compare the calculated transmission between crystalline and amorphous states, respectively, when varying the period P and diameter D. This plot shows obvious transmission contrast between the two states. In the crystalline state, most light transmits through the film, thus we call this ON state. By contrast, in the OFF state, the normalized transmission spectra are sharp and quite lower when the GST is amorphous. In addition, the frequency selectivity of the transmitted light means that this PC-AOS can provide a wide tunability across the near infrared spectrum. From the perspective of Fano resonance, the holes-induced eigenmodes and quasi-continuous cavity mode play the roles of the narrow resonance and broad resonance, respectively. The Fano line shape is the result of interference between them. The electric field distributions of crystalline and amorphous states are simulated by the Lumerical Finite Difference Time Domain (FDTD) Solution, as shown in Figure 2C. Apparently, in the spacing layer between the GST and Si, there exists strong electric field confinement owing to Fano resonance when GST is amorphous. And the field enhancement effect occurs at the resonance wavelength, which is slight at crystalline state.

Device image and switching characteristics
We next design and experimentally demonstrate a phase change optical switch at 1550 nm wavelength. After electronic beam exposure, we etched the silicon substrate filmed with ZEP520 photoresist for submicron holes by inductively coupled plasmon (ICP) technique. From the scanning electron microscope (SEM) image in Figure 3A, the diameter of the submicron holes is 150 nm, and the distance between each two-unit cell is 810 nm. The transmission electron microscope (TEM) image of the entire device in iScience Article Figure 3B confirms that the depth of the submicron holes is approximately 210 nm. GST is amorphous when it is deposited with magnetron-sputtering technique. The TEM image of the first three layers is shown in Figure 3C. The atoms' lattice symmetry can be found in the GST layer. The device in a square area of 81 3 81 mm 2 has 10,000 (100 by 100 array) submicron holes in total.
To improve the extinction contrast of the device, an extremely thin SiO 2 film beyond PhCs was deposited to lower the environmental dielectric properties and then an amorphous GST layer of 15 nm was sputtered above. On top of the GST film, there was a capping layer to isolate GST from air, otherwise it would be easily evaporated or oxidized when we perform phase transition with laser pulse or annealing. We choose ZnS/SiO 2 which is oxidation-proof as the capping layer. This capping layer can also decrease the possibility of material loss (  To identify the direction of phase transition, we first annealed the sample to get a complete crystalline state and then focused on the re-amorphization. The phase transition of GST is a thermodynamic process. The crystallization temperature (T c ) and melting temperature (T m ) of GST are reported to be 160 C and 600 C (Terao et al., 2009) (He et al., 2014). To achieve full crystallization especially in a large area, the heating process takes a relatively long time and the temperature has to be kept beyond T c and below T m . It is, however, much easier for a small area to change its phase by laser pulse stimulation. A complete crystallization phase transition can be induced by a series of laser pulses or single pulses with particular energy and time duration (Michel et al., 2014) (Behrens et al., 2019). In addition, dielectric materials are different from metallic materials in thermal accumulation. The process from crystalline state to amorphous state requires higher energy. An all-dielectric structure may have a larger influence on this process.
Because the as-deposited GST is amorphous, the sample is annealed in a furnace at 250 C for an hour. The transmission is characterized using a near-infrared spectrum microscope, as shown in Figure 3D. When GST is amorphous, a deep and sharp valley near 1550 nm is observed. After annealing, the valley has red-shifted, and the depth is shallower than the former because the refractive index increases when GST turns into the crystalline state. The loss of the device is measured as 2.8 dB, which is lower than the loss of switching based on plasmonic metamaterials (Gholipour et al., 2013) (Kang et al., 2021). The simulation results are shown in Figure 3E. It is slightly different from the measurement (such as transmittance, peak position), which is because of the size vibration and the optical constant differences of the material between fabrication and simulation. The switching contrast is calculated to be 5.4 dB near 1550 nm by comparing the ON-state and the OFF-state spectra from Figure 3D. The contrast is higher than other similar works, and an even higher contrast can be obtained at 1583 nm which is 7.4 dB, as seen in Figure 3F. We next performed a femtosecond laser induced re-amorphization for the PC-AOS devices. Phase transformation from amorphous to crystalline state can be achieved by both annealing and a wide laser pulse with particular power. In comparison, the re-amorphization (from crystalline state to amorphous state) of GST is quite demanding, because the crystal lattice needs to be molten and rapidly cooled to room temperature to avoid crystallization. Hence, a narrow pulse with higher power can switch it back to glass (Boltasseva and Atwater, 2011). However, when the pulse width is only femtosecond or sub-picosecond scale, the thermal energy is easily dissipated, as a consequence, the temperature is not the only reason that is responsible for the phase transition (Hu et al., 2015) (Huang et al., 2011).
To re-amorphize the GST, a sequence of femtosecond laser pulses is generated as the ''control light''. But if the pulse energy is accumulated in a small area, the localized heat could rise quickly, and the internal stress may destroy the capping layer. To avoid this, we controlled the pulse number by shifting the laser probe. We set the laser energy density to 35 mJ/cm 3 , and the pulse band is 45 fs with the repetition frequency of 1 kHz. The wavelength of the laser beam is 1990 nm. Reducing the number of pulses applied on the phase change material was achieved by moving a femtosecond laser rapidly. Because the light spot diameter is around 100 mm, the speed of the pump probe moving is 0.61 mm/ms on average (the method is described in Figure 4A). Approximately 100 pulses are applied to the device within a distance of 100 mm. Considering there is a path of acceleration, the beginning is offset by several millimeters away from the testing region. The light trace can be observed with human eyes after laser exposure. We then realize the cycling characteristics of the switch using the aforementioned method. Two ON/OFF cycles are shown in Figure 4B. It started as the same as Figure 3D where GST was as-fabricated amorphous. Then the crystalline state of GST was obtained after annealing. By moving the laser probe, the pulseinduced re-amorphous GST results in a spectrum shifting that is similar to the initial state. Then we repeated this cycle again. The spectrum turns back to the former. The switching time was determined as around 4.5 ps.
The change of optical properties in chalcogenide materials is usually caused by structure transition (Hosokawa et al., 2012). The atoms have been re-aligned from amorphous to crystal structure, whereas the reversed process requires high energy to get a melt-quenched amorphous state. However, in the picosecond timescale pulse-induced phase change, it is considered that the atoms have no time to recombine. According to recent research, the large difference in dielectric function between amorphous and crystalline GST is caused by resonant bonds in the crystalline state (Shportko et al., 2008) (Huang and Robertson, 2010). It is reported that femtosecond optical excitation can instantaneously break the resonant bonds in crystalline state (Waldecker et al., 2015), leading to a drop of dielectric function when the phase is changed. As more of such bonds are depopulated, the larger percentage change of dielectric function can be achieved (Shportko et al., 2008). When the sub-picosecond or femtosecond laser pulses are appealed to GST, the resonant bonds are broken and the optical property changes, such as the re-amorphization process in Figure 4B. The laser power not only breaks the resonant bonds instantaneously but changes the optical properties before ionic motion has occurred. The remaining energy could also heat the lattice, which thermally melts the long-range order after several picoseconds (Waldecker et al., 2015). So that GST can be reamorphized in picoseconds. We indicate this mechanism in Figure 5. Because of the rapid heating, it is very probable that the laser re-amorphizes the materials without melting (Kolobov et al., 2011) (Nam et al., 2012). Figure 5D is the TEM image after annealing and represents the crystalline GST iScience Article as in Figure 5A. The crystalline lattices are arranged orderly in the GST layer. After the laser exposure, the TEM image shows a mixture of amorphous and crystalline phases in Figure 5E. This is because of the heating by the residue energy of laser pulses after breaking the resonant bonds ( Figure 5C).
Regarding the picosecond-level switching speed shown by the structure proposed in this paper, we have considered a reasonable interpretation of this result from the perspective of non-thermal effects in (Waldecker et al., 2015). Artificial metal arrays will extract the energy used to depopulate the resonant bonds before the lattice heats above the T m . Energy extraction could be realized in nanostructured devices by rapid transfer of the photoexcited carriers, both electrons and holes, into a metal or semimetal, and resonant bonds could re-establish and recover the crystalline-state optical properties on the few-picosecond timescale. We believe that the all-dielectric structure inhibits the recovery process of the resonant bond, so a faster switching speed has been achieved than the metallic metamaterial all-optical switch.
Then we compare the proposed PC-AOS with other existing works. Table 1 summarizes various optical switch designs based on phase change materials. The proposed all-dielectric PC-AOS achieves 2.8 dB loss which is lower than most plasmonic all-optical switches. The switching contrast is 5.4 dB for a 1550 nm laser, which is more than enough for optical communication device applications. In addition, the thickness of the GST active layer used in the optical switch designed in this paper is thinner, which means that the phase change process can be achieved by using a laser with a lower energy level. In addition, the proposed all-dielectric PC-AOS can be switched at picosecond timescale, which is comparable with the other all-optical switch systems, such as Mach-Zehnder based and Kerr effect based ones. A thinner GST film also enhances the switching contrast of this device. The SiO 2 film between GST and PhCs can decrease the average surface refractive index so that the switching contrast of the device can be further increased.

CONCLUSION
In summary, an freespace all-optical switch has been demonstrated by incorporating the phase change material with low loss all-dielectric metamaterials. By altering the structural parameters, the PhCs show the switching property in different wavebands. We have confirmed that the switching function can be realized by femtosecond laser. The proposed PC-AOS features a multi-film coated structure based on SOI iScience Article substrate, which adapts to modern CMOS integrated technique. Thus it enables massive manufacture possible. Integrated with optical waveguide, the device can be more practical for all-optical communication. The signal transmission of ''node to node mode'' in optical communication can be safer and more efficient. Theoretically, the cycling of GST film can achieve trillions of times, which can be superior to some other similar devices. This work also provides a platform for researching the physical mechanism of metavalence.

Limitations of the study
In this study, we have simulated, fabricated and characterized GST-225 based all optical switch, which can be working at freespace. The switching property can occur at various wavebands by fine-tuning the periods and diameters of nanoholes. In this design, we use a periodically arranged dielectric nano-hole array, which is symmetrical. Thus, the proposed device does not depend on the polarization of the signal light. In addition, we use only 15 nm GST as the active layer, which is more favorable for light control. Using all-dielectric materials enables the PC-AOS, thus showing a lower loss than other existing designs. In addition, we carried out laser induced switching experiments for investigating the reversible ON/OFF property. However, only several cycles have been performed currently; one could perform a longtime endurance test to demonstrate the cycling ability.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:

ADDITIONAL RESOURCES
Any additional information about the simulation and data reported in this paper is available from the lead contact on request.
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